OCI image spec 1.1.0-rc4 allows image index to have subject(updated in this commit) and artifactType(update in this commit). Since both fields are optional, it's possible that user can push an index with subject but no artifactType like below.

flowchart
index --subject--> image

My questions is: should such index be considered as valid when being pushed? If yes,

1. When Referrers API is enabled and user is trying to get the referrers of image, should the descriptor of index be included in the returned value of Referrers API?
2. When Referrers API is not enabled, client need to add a descriptor to the referrers index, what should the artifactType be in the newly added descriptor? Distribution-spec 1.1.0 rc3 only covers image referrer but not mentioning index.